The present invention relates generally to a composite airfoil structure for aircraft engines and more particularly to a method and system for forming a composite airfoil structure without incurring wrinkles and porosity defects.
Presently, a composite airfoil structure (i.e., a fan blade) can be formed by placing a resin impregnated preform of the blade in a mold having a stationary lower mold piece and a vertically moving upper mold piece. As the upper mold piece closes towards the lower mold piece, the mold forms a cavity with all fours sides of the preform (e.g. root, tip, leading edge and trailing edge sides) closed. In this approach, the mold first contacts the root section of the preform and moves towards the tip section of the preform last. The moving contact and the closed mold cause the resin to flow mainly in the blade in a lengthwise direction. Since the resin flows in the lengthwise direction, it will exert a shear stress on the plies of the preform. When the shear stress overcomes the interfacial friction between adjacent ply layers, the plies will move in the direction of the net resultant shear force, causing the plies to wrinkle. In addition, the mold force, the heat transfer, and the chemical reaction of the compression molding process is unsymmetrical and difficult to predict and control because of the progressively changing contact between the mold and the preform. The unpredictability often results in porosity defects.
To overcome the disadvantages associated with the moving contact of the compression molding process, a fan blade can be formed with a bag molding process, a constant pressure process, which uses one stationary mold surface and a flexible upper inflatable bag surface. Although the bag molding process does provide constant contact, it cannot provide the same amount of force and heat, or the same quality of dimension, surface or mechanical properties associated with the moving contact of the constant displacement compression molding process. Thus, there is a need for a method and a system for forming a blade that provides full constant contact between the mold and the preform as with the bag molding process and that provides the same amount of force and heat as with the moving contact of the compression molding process.